KR101229824B1 - Photocatalysts for water splitting and synthesis methods thereof - Google Patents

Abstract

The present invention relates to a photocatalyst for water decomposition and a method for producing the same, wherein electrons and holes generated from the photocatalyst under light irradiation may decompose pure water to generate hydrogen.

Description

Photocatalysts for water splitting and synthesis methods

The present invention relates to a photocatalyst for water decomposition and a preparation method thereof.

In general, the term photocatalyst is used so widely that it is difficult to find a consonant definition. However, it is a very broad definition, and in order to be a 'photocatalyst', it is generally required to satisfy the condition of 'catalyst'. Used to refer. In other words, the photocatalyst should not be consumed by directly participating in the reaction, but by accelerating the reaction rate by providing a mechanism mechanism different from the existing photoreaction. The ratio of the product per active site (this is defined as turnover ratio). Must exceed 1.0. In the case of photocatalysts, it is difficult to accurately measure the number of active sites, so the ratio of the product per active site (defined as turnover ratio) cannot be determined. The approximate value calculated on the assumption that the reaction occurs at / cm ² ) is reported to be greater than 1.0.

Such photocatalysts require light energy above the band gap energy (Eg) to exhibit photochemical activity, which is not occupied by the valence band (VB), the highest band of energy occupied by the electron. The difference in conduction band (CB), which is the lowest energy band, is a forbidden interval that electrons cannot occupy, and it is the minimum that can excite the electrons in the covalent band to create an electron / hole pair that participates in the reaction. Is energy. These bandgaps have a continuous energy level, which prevents the rapid recombination of electrons / holes, unlike metals with extremely fast electron / hole recombination.

Meanwhile, since hydrogen energy does not cause any by-products other than water during combustion, it has attracted great attention as a future energy source. In addition, the water decomposition reaction technology using a photocatalyst decomposes water using oxygen and hydrogen in the electric conduction band of holes and conduction bands generated when the photocatalyst having a band gap energy is irradiated with light having an energy above the band gap. As a technology for producing, such a water decomposition reaction technology is receiving great attention because it can directly obtain hydrogen energy, a clean energy source, from abundant water resources and solar energy. A photocatalyst which exhibits excellent water-splitting activity under ultraviolet light is _{K 4 Nb 6 O 17, BaTiO} 4, NaTaO 3, La 2 Ti 2 O 7 and the like, in particular La ₂ Ti ₂ O ₇ having a layered perovskite structure, Excellent activity due to its unique electronic structure (Kim, HG Hwang, DW Kim, J. Kim, YG Lee, JS Chem. Commun. 1999, 1077; Hwang, DW Kim, HG Kim, J. Cha, KY Kim , YG Lee, JSJ Catal. 2000, 193, 40.). Nevertheless, in order to apply the water decomposition technology using a photocatalyst from a practical point of view, the development of a photocatalyst with better activity is required.

An object of the present invention is to provide a photocatalyst for water decomposition and a method for producing the same, wherein the electrons and holes generated from the photocatalyst under light irradiation decompose pure water to generate hydrogen.

One embodiment according to the present invention is a photocatalyst for water decomposition represented by the following Chemical Formula 1.

≪ Formula 1 >

Zn ₃ M ₂ O ₈

(Wherein M is Nb or Ta)

Another embodiment according to the present invention is a photocatalyst for water decomposition, characterized in that the photocatalyst supports any one metal element selected from Pt, Pd, Ag, Ru, and Ni.

According to another embodiment of the present invention, the metal element is a photocatalyst for water decomposition, characterized in that supported by 0.1 to 1% by weight relative to the photocatalyst.

Another embodiment according to the present invention is a method of preparing a photocatalyst for water decomposition to prepare a compound represented by the following Chemical Formula 1 by pulverizing and mixing ZnO and a metal oxide.

≪ Formula 1 >

Zn ₃ M ₂ O ₈

(Wherein M is Nb or Ta)

Another embodiment according to the present invention is a method for producing a photocatalyst for water decomposition, characterized in that the metal oxide is Nb ₂ O ₅ or Ta ₂ O ₅ .

Another embodiment according to the present invention is a method for preparing a photocatalyst for water decomposition, characterized in that the mixture is weighed and mixed so that the molar ratio of ZnO to metal oxide is 3: 1.

Another embodiment according to the present invention is a firing method for producing a photocatalyst for water decomposition, characterized in that carried out for 2 to 10 hours at 1100 to 1300 ℃.

Another embodiment according to the present invention further comprises the step of sterling by immersion in the compound solution having any one metal element selected from Pt, Pd, Ag, Ru and Ni after the firing and then irradiated with light It is a manufacturing method of the photocatalyst for water decomposition characterized by the above-mentioned.

The photocatalyst according to the present invention can be sensitive to visible light, and can generate hydrogen due to the photoexcitation of electrons and holes to decompose water, and has the advantage of using abundant and cheap zinc-based as a starting material on the earth. .

1 is Zn ₃ M ₂ O ₈ of the present invention (M = Nb, Ta) shows a photocatalyst crystal structure ((a) Zn ₃ Nb ₂ O ₈ ; (b) Zn ₃ Ta ₂ O ₈ ).
FIG. 2 is a graph showing electron band structures and density of states (DOS) of a Zn ₃ M ₂ O ₈ photocatalyst ((a) Zn ₃ Nb ₂ O ₈ ; (b) Zn ₃ Ta ₂ O ₈ )
3 is an XRD and FESEM photograph of the Zn ₃ Nb ₂ O ₈ (1100 ° C.) photocatalyst according to the present invention.
4 is an XRD and FESEM photograph of a Zn ₃ Ta ₂ O ₈ (1300 ° C.) photocatalyst according to the present invention.
5 is Zn ₃ M ₂ O ₈ according to the present invention (M = Nb, Ta) is a graph measuring the (a) UV-Vis diffuse reflectance spectra and (b) optical band gap of the photocatalyst.
6 is Zn ₃ M ₂ O ₈ according to the present invention It is a graph showing Mott-Schottky plots of thick film electrodes prepared using (M = Nb, Ta) photocatalyst.
7 is Zn ₃ M ₂ O ₈ according to the present invention (M = Nb, Ta) A graph showing the potentials of the conduction band and the covalent band of the photocatalyst.
8 is Zn ₃ M ₂ O ₈ according to the present invention (M = Nb, Ta) is a graph showing a hydrogen generation under UV irradiation of the photocatalyst _{((a) Zn 3 Nb 2} O 8 and Zn containing Pt cocatalyst _{3 Nb 2 O 8 (b)} Zn 3 Ta 2 O 8 And Zn ₃ Ta ₂ O ₈ ) comprising a Pt cocatalyst.
9 is Zn ₃ M ₂ O ₈ according to the present invention (M = Nb, Ta) A graph measuring XRD before and after photocatalytic reaction of photocatalyst ((a) and (b): Zn ₃ Nb ₂ O ₈ , (c) and (d): Zn ₃ Ta ₂ O ₈ ) .

According to an embodiment of the present invention to provide a photocatalyst for water decomposition represented by the following formula (1).

≪ Formula 1 >

Zn ₃ M ₂ O ₈

(Wherein M is Nb or Ta)

As the photocatalyst for water decomposition, which is a compound represented by Chemical Formula 1, it can be seen that Nb-O or Ta-O octagons are arranged in a layered structure in the crystal. These Nb-O octagonal Ta-O octagons have a great influence on the formation of the crystal's electronic structure, and the 2p orbits of oxygen in the highest occupied molecular orbital (HOMO) are most involved. At the bottom (LUMO: Lowest unoccupied molecular orbital), the d-orbit of Nb or Ta is most involved. In addition, the photocatalyst for water decomposition has a positive value greater than 1.23 V compared to a normal hydrogen electrode (NHE) as shown in FIG. 7, and the conduction band has a sufficiently negative value than 0 V. Water can be decomposed under induction to effectively generate hydrogen.

As for the photocatalyst, it is more preferable to support any one metal element selected from Pt, Pd, Ag, Ru, and Ni.

The metal element acts as a co-catalyst, and when supported on the photocatalyst, photoexcited electrons and holes can be easily moved to obtain an effect of further improving hydrogen generation.

The metal element is preferably supported by 0.1 to 1% by weight based on the photocatalyst.

According to another embodiment of the present invention, ZnO and metal oxides are pulverized and mixed, and then calcined to provide a method for preparing a photocatalyst for water decomposition to prepare a compound represented by the following Chemical Formula 1.

≪ Formula 1 >

Zn ₃ M ₂ O ₈

(Wherein M is Nb or Ta)

First, the raw material ZnO and the metal oxide are pulverized.

The metal oxide is Nb ₂ O ₅ or Ta ₂ O ₅ .

The raw material is pulverized and mixed. At this time, it is preferable to mix and measure the molar ratio of ZnO to metal oxide to be 3: 1.

Subsequently, the mixed raw materials are fired at 1100 to 1300 ° C. for 2 to 10 hours to prepare a photocatalyst as a compound represented by Chemical Formula 1.

At this time, the step of immersion in the compound solution having any one metal element selected from Pt, Pd, Ag, Ru and Ni after the firing and then irradiated with light may be further performed.

In detail, for example, when platinum (Pt) is supported in the metal element, 0.5 wt% of platinum is used as a photocatalyst for H ₂ PtCl ₆ .6H ₂ O, a compound solution having a photocatalyst and platinum in a beaker of Pyrex material. Platinum may be supported on the photocatalyst by photodeposition by weighing the compound solution and the photocatalyst so that they are loaded with% and stirring for 10 hours while irradiating light using a mercury lamp.

When the metal element is supported on the photocatalyst by carrying out the step of supporting the metal element, a cocatalyst may easily move electrons and holes generated by the photocatalyst, thereby further improving hydrogen generation. .

Hereinafter, preferred embodiments and comparative examples of the present invention will be described. However, the following embodiments are merely preferred embodiments of the present invention, and the present invention is not limited to the following embodiments.

< Example  1>

ZnO and Nb ₂ O ₅ powders were ground by ball milling (zirconia media) for 24 hours, and then weighed and mixed such that the molar ratio of ZnO to Nb ₂ O ₅ was 3: 1.

The mixture was calcined at 1100 ° C. for 10 hours to prepare a photocatalyst of Zn ₃ Nb ₂ O ₈ .

< Example  2>

ZnO and Ta ₂ O ₅ powders were ground by ball milling (zirconia media) for 24 hours, and then weighed and mixed such that the molar ratio of ZnO to Ta ₂ O ₅ was 3: 1.

The mixture was calcined at 1300 ° C. for 10 hours to prepare a photocatalyst of Zn ₃ Ta ₂ O ₈ .

< Example  3>

The Zn ₃ Nb ₂ O ₈ photocatalyst prepared according to Preparation Example 1 and the H ₂ PtCl ₆ .6H ₂ O solution were weighed into the compound solution and the photocatalyst so that platinum was supported by 0.5 wt% with respect to the photocatalyst, and the photocatalyst was immersed in the compound solution. After that, the Pt was supported on the surface of the photocatalyst by stirring with light using a mercury lamp for at least 10 hours.

< Example  4>

The Zn ₃ Ta ₂ O ₈ photocatalyst prepared according to Preparation Example 2 and the H ₂ PtCl ₆ .6H ₂ O solution were weighed into the compound solution and the photocatalyst so that platinum was supported by 0.5 wt% with respect to the photocatalyst, and the photocatalyst was immersed in the compound solution. After that, the Pt was supported on the surface of the photocatalyst by stirring with light using a mercury lamp for at least 10 hours.

Photocatalysts prepared according to Examples 1 and 2 were measured as follows.

1. Crystal structure was measured using an X-ray powder diffractometer (XRD; Macscience, M18XHF, Yokohama, Japan).

2. Morphologies were measured using filed-emission scanning electron microsope (FESEM; Jeol, JS-6330F, Tokyo, Japan).

3. Surface area analysis was performed using Brunauer-Emmett-Teller (BET) surface area analyzer (ASAP 2010, Micromeritics Instrument Corporation, Norcross, GA).

4. Diffuse reflectance spectra were measured using a UV-VIS spectrophotometer (Model Lamda 35, Perkin Elmer, United Kingdom).

5. Photoelectrochemical characterization is achieved using potentiostat (CHI 608A) using a conventional three-electrode cell with a Pt counter electrode and a saturated calomel reference electrode (SCE). , CHInstruments).

6. Photocatalytic activity was measured by gas chromatograph (GC) while irradiating a high pressure mercury lamp while putting a photocatalytic material with water in a reactor and stirring.

Electronic strip  rescue( Electronic band structures ) analysis

1 is Zn ₃ M ₂ O ₈ of the present invention (M = Nb, Ta) shows a photocatalyst crystal structure ((a) Zn ₃ Nb ₂ O ₈ ; (b) Zn ₃ Ta ₂ O ₈ ).

As shown in Figure 1, it can be seen that the Nb-O or Ta-O octagonal structure is present.

FIG. 2 is a graph showing electron band structures and density of states (DOS) of a Zn ₃ M ₂ O ₈ photocatalyst ((a) Zn ₃ Nb ₂ O ₈ ; (b) Zn ₃ Ta ₂ O ₈ )

As shown in Figure 2, it can be seen that Nb-O or Ta-O is deeply involved in the electronic structure.

From this, it can be seen that the Nb 4d orbital and Ta 5d orbital affect the conduction band of Zn ₃ M ₂ O ₈ (M = Nb, Ta) to improve the photocatalyst characteristic of generating hydrogen. . In most cases, for oxides, the conduction band is not sufficient to decompose water to generate hydrogen, but Zn ₃ M ₂ O ₈ (M = Nb, Ta) according to the present invention is high due to the contribution of the conduction band of Nb and Ta. It has a reducing power, which is advantageous for generating hydrogen by photosensitization.

Photocatalyst  Crystal structure analysis

FIG. 3 shows XRD and FESEM photographs of a Zn ₃ Nb ₂ O ₈ (1100 ° C.) photocatalyst according to the present invention. FIG. 4 shows XRD and FESEM of a Zn ₃ Ta ₂ O ₈ (1300 ° C.) photocatalyst according to the present invention. It is a photograph. All reflection peaks are Zn ₃ Nb ₂ O ₈ Monoclinic phase and Zn ₃ Ta ₂ O ₈ (JCPDS No.50-1725) (JCPDS No. 52-1764) found in the monoclinic, no other phases observed.

Optical absorption ( Optical absorption Characterization

5 is Zn ₃ M ₂ O ₈ according to the present invention (M = Nb, Ta) is a graph measuring the (a) UV-Vis diffuse reflectance spectra and (b) optical band gap of the photocatalyst.

As shown in FIG. 5, Zn ₃ Nb ₂ O ₈ 295nm and Zn ₃ Ta ₂ O _{8 in} Photocatalyst It can be seen that absorption edges appear at 269 nm of the photocatalyst. From this electronic band Zn ₃ M ₂ O ₈ It can be seen that the (M = Nb, Ta) photocatalyst effectively absorbs light.

Surface and band dislocation characterization

6 is Zn ₃ M ₂ O ₈ according to the present invention It is a graph showing Mott-Schottky plots of thick film electrodes prepared using (M = Nb, Ta) photocatalyst. The Mott-Schottky equation is

1 / C ² _{= (2 / ε 0 ε s} qN D) [VV fb - (k B T / q)]

Where C is the capacitance between the electrode / electrolyte interface and V is the voltage of the electrode between the electrode / electrolyte interface. And ε ₀ and ε _s are the dielectric constants of free space and thin film electrode, q is charge amount, V _fb is flat band potential value, K is temperature, ND is donor density, V is applied voltage, kB is Boltzmann constant It means. Where Vfb is the value of x-intercept when the extension line of the measured graph is 1 / C ² = 0.

As shown in FIG. 6, all of the flat band potential values have negative values, and the photocatalyst has characteristics of n-type semiconductors.

7 is Zn ₃ M ₂ O ₈ according to the present invention (M = Nb, Ta) A graph showing the potentials of the conduction band and the covalent band of the photocatalyst. The electric potential of the conduction band and the covalent band was calculated by electronegativity. The following equation was used.

E _vb = EN-E ^e + 0.5 × Eg

Where E _vb is the potential of the covalent band. EN was obtained from the geometric mean of the electronegativity of each constituent atom as the electronegativity of the material. E ^e represents the energy of free electrons at the standard hydrogen potential. Finally, E g is calculated by analyzing the light absorption characteristics measured by UV-Vis spectrometer with band gap.

As shown in FIG. 7, the covalent band of the photocatalyst for water decomposition has a positive value greater than 1.23 V compared to a normal hydrogen electrode (NHE), and the conduction band has a sufficiently negative value greater than 0 V, under light sensitivity. By decomposing water, hydrogen can be generated effectively.

Photocatalyst  Activity analysis

8 is Zn ₃ M ₂ O ₈ according to the present invention (M = Nb, Ta) is a graph showing a hydrogen generation under UV irradiation of the photocatalyst _{((a) Zn 3 Nb 2} O 8 and Zn containing Pt cocatalyst _{3 Nb 2 O 8 (b)} Zn 3 Ta 2 O 8 And Zn ₃ Ta ₂ O ₈ ) comprising a Pt cocatalyst.

As shown in FIG. 8, it can be seen that hydrogen is effectively generated under light irradiation.

9 is Zn ₃ M ₂ O ₈ according to the present invention (M = Nb, Ta) A graph measuring XRD before and after photocatalytic reaction of photocatalyst ((a) and (b): Zn ₃ Nb ₂ O ₈ , (c) and (d): Zn ₃ Ta ₂ O ₈ ) .

As shown in FIG. 9, it can be seen that after the light irradiation reaction, the photocatalyst effectively performs the role of the photocatalyst without changing the phase.

Photocatalyst  Characterization conclusion

As can be seen from the above-described photocatalytic characterization of the photocatalyst, electrons and holes were generated under light irradiation to effectively decompose pure water to generate hydrogen. In addition, when the Pt cocatalyst was supported, it was observed that the photocatalytic activity was improved.

All simple modifications or changes of the present invention can be easily carried out by those skilled in the art, and all such modifications or changes can be seen to be included in the scope of the present invention.

Claims (8)

The photocatalyst for water decomposition represented by the following formula (1).
&Lt; Formula 1 >
Zn ₃ M ₂ O ₈
(Wherein M is Nb or Ta) The photocatalyst for water decomposition of claim 1, wherein the photocatalyst carries one metal element selected from Pt, Pd, Ag, Ru, and Ni. The photocatalyst for water decomposition of claim 2, wherein the metal element is supported in an amount of 0.1 to 1 wt% based on the photocatalyst. A method of producing a photocatalyst for water decomposition in which ZnO and a metal oxide are ground and mixed, and then calcined to produce a compound represented by the following Chemical Formula 1.
&Lt; Formula 1 >
Zn ₃ M ₂ O ₈
(Wherein M is Nb or Ta) The method of claim 4, wherein the metal oxide is Nb ₂ O ₅ or Ta ₂ O ₅ . The method for producing a photocatalyst for water decomposition according to claim 4, wherein the molar ratio of the ZnO to the metal oxide is 3: 1 and mixed. The method of claim 4, wherein the firing is performed at 1100 to 1300 ° C. for 2 to 10 hours. 5. The method of claim 4, further comprising the step of immersion in a compound solution having any one of metal elements selected from Pt, Pd, Ag, Ru and Ni after light firing, followed by sterling. Method of producing a photocatalyst for water decomposition.

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